The invention relates to a circuit arrangement for the location of faults, which operates in conjunction with equipment for trunk feeding of electrical loads.
German patent DE-AS No. 1 157 663 discloses a circuit arrangement in which a resistance is connected in series with a diode whose poles are inverse to the supply voltage between the trunk supply current paths at each waystation. In case of a fault, the input resistance of the remaining line section is measured at the supply point with a harmless voltage of, for example 60 V which is polarized inverse to the trunk supply voltage. Conclusions as to the location of the interrupted field can be drawn from the current. However, instead of test resistors, German patent DE-AS No. 2 620 348 discloses that constant current diodes may be provided in the waystations.
The diodes in parallel to the loads may be formed by one or more series-connected diode paths or by Z-diodes; and if applicable, these diodes serve to limit or stabilize the load voltage. When the loads are not adequately protected against supply voltages of the wrong polarity by the parallel-connected diodes, diodes connected in series to the loads may be installed additionally. The polarity of these additional diodes is the forward direction of the trunk supply current. Such an arrangement is described in German patent DE-AS No. 1 157 663.
British patent application Ser. No. 1,286,033 discloses the use of switching ancillaries for the trunk supply of electrical load equipment for the automatic closing of the trunk supply loop ahead of a point of interruption. These devices close the trunk supply loop across a cross branch as long as no current of a given magnitude is measured in the continuing part of the trunk supply loop. In this manner one section of the trunk supply line after another is tested and, if faults are not detected, put on line when the installation is put into operation. This makes it easy to avoid putting the installation into operation if there is a resistance approaching or even exceeding a given resistance value corresponding to the human body resistance at a point of interruption.
German patent application DE-AS No. 1 154 525 describes another embodiment of equipment to close a trunk supply loop ahead of a point of interruption.
On the other hand, equipment for the trunk supply of electrical loads can be constructed by means of dc series feeding without such switching ancillaries, when there is only limited mounting space. When putting such trunk supply lines into service the trunk supply equipment at the supply point must lock the trunk supply current onto the loop resistance of the entire line. However, for safety reasons such a lock-on must not occur when the trunk supply circuit is closed at a point of interruption due to contact via human body resistance.
As long as the trunk supply current is so low that it does not exceed a value permissible in the case of human contact (for example 40 mA), the lock-on can proceed without additional measures.
However, at high trunk supply currents it is expedient to check before the start-up whether the supply loop is completely closed rather than bridged due to contact at a point of interruption. This entails the problem that the loop resistance of a complete trunk supply line, particularly at low test voltages or currents, is by far greater than the human body resistance which can be assumed to equal 2000 Ohms. Consequently, difficulties may result when testing the entire trunk supply loop, because a good loop cannot be distinguished with sufficient certainty from one bridged at a point of interruption by human body resistance. It is particularly disadvantageous that in view of the desired test, the trunk supply range and the specific resistance of the trunk supply current paths are limited.
A method for starting up the trunk supply of electric loads which ensures a safe, in particular automatic, start-up of a trunk supply line without equipment for the automatic closing of the trunk supply loop ahead of a point of interruption regardless of the load resistance, involves the following steps.
One resistance of the trunk supply is tested first and the trunk supply current is turned on only after the trunk supply loop resistance has been found at the supply point to be below a specified value. The trunk supply loop resistance testing is conducted with the current flowing during the test having the opposite direction with regard to the trunk supply current.
In one embodiment of a circuit arrangement proposed for the implementation of this method there is provided at the supply point an instrument for measuring the trunk supply current and the test current.